A gas turbine is known to comprise a rotor or turbine shaft which is rotatably supported by a bearing, a compressor including rotor blades mounted at the rotor to compress air, a combustor for burning a mixture of fuel and the compressed air, and a turbine portion which converts the energy contained in the burned mixture of fuel and compressed air into mechanical energy to drive the rotor. A gas turbine may for example be used to generate electric energy by driving a generator using the mechanical energy.
From US 2007/0107438 A1 a gas turbine is known, wherein the rotor is rotatably supported by bearings including lubricating oil. Seal rings are provided near the bearings so that the lubricating oil does not flow out of the bearings. In addition, seal air which is extracted from the compressor is supplied to the seal rings so that the lubricating oil does not leak out of the bearings. The turbine compressor or a supplementary air source may be utilized to provide the seal air. A valve arrangement is provided for switching between the gas turbine internal air source and the supplementary air source.
GB 702,931 discloses a rotary machine, wherein air bled from the high pressure compressor is used to exclude lubricating oil from the air flow passages of the compressor.
GB 2 111 607 A discloses a bearing chamber pressurization system, wherein a pressure sensing valve controls the air supply to the bearing from two sources having different pressure levels within a gas turbine engine.
EP 0 354 422 B1 discloses a gas turbine, wherein a valve controls the air pressure in the turbine bearing chamber as a function of representative parameters of the operation condition.
FR 2 698 406 discloses a procedure for pressurizing the lubrication chamber of a turbine, wherein the flow is controlled by a valve.
It has been observed that a conventional turbine does not ensure under all operation conditions of the turbine that lubricating oil contained in a bearing chamber for bearing the rotor is prevented from escaping the bearing chamber towards other components of the turbine. Further, it has been observed that under certain operation conditions the demand of air provided by an external compressor is relatively high which adds significant costs and also reduces the efficiency of the overall system.
An object of the present invention is to provide a turbine having a smaller demand on seal air from an external compressor, thus having a higher efficiency, and which at the same time ensures that no bearing fluid, in particular oil, escapes from the bearing chamber.
Another object of the present invention is to provide a turbine improved in particular during certain operation conditions, such as hot shut-down, where the turbine is rapidly shut-down for example by shutting off the fuel supply in the case of an operational failure.